The silk road to epigenetics

Silkworms are under the spotlight after the publication of a study where epigenetic modifications are compared between wild and domesticated varieties. This work unravels the mechanisms that led to the domestication of silkworms and the improvements in silk production about 5000 years ago in China.

The domesticated silkworm, Bombyx mori, was the focus of the study by Xiang and colleagues, published online on the 23rd of September 2013 in the journal BMC Genomics. Photo: Wikipedia

The domesticated silkworm has been artificially selected to produce more and better silk, the luxurious product that is so valued worldwide. It has been estimated that silk production has increased up to tenfold with sericulture (the breeding of silkworms for the harvesting of silk, also known as silk farming). For example, the domesticated variety shows nowadays a series of human-preferred traits such as faster growth, bigger cocoons and increased resistance to disease than the wild variety. Silk production is of great economical importance, especially in rural areas of the main producing countries, China and India, and research in these animals is expected to lead to an important increase in revenues. In addition, these are good model organisms for studies in invertebrates, as they easy to breed and have fast growth. Now, a team comprising scientists from various Institutes in China has asked the question: which are the main differences found in domesticated silkworms, apart from genetic divergences, that contribute to their particular characteristics?

In a paper just published, the authors claim that epigenetic divergences were likely to contribute decisively to the process of silkworm domestication. It has long been known that domestication led to the deviation of the populations – wild and bred by farmers – from each other to such a degree that the two varieties can no longer mate to produce fertile offspring, only hybrids. However, factors other than alterations in the genetic code may also contribute to the appearance of new characteristics in animals. These are called epigenetic modifications and do not alter the sequence of the bases in the genome, only the way that particular genes are converted into traits (gene expression). The new study has established that epimutations in the domesticated varieties of silkworms, in the same way as mutations, may have given rise, for example, to bigger salivary glands, the producing organs of silk.

The study was comprehensive in that it focused on the entire epigenome of wild versus domesticated individuals. One particular epigenetic modification, methylation of DNA bases (DNAme), was chosen for comparison between the wild and domesticated populations. A variety of genes were found to have differences in epigenetics, according to results obtained from sequencing the entire genomes in the search for the exact patterns of DNAme. The fact that more than genetics has contributed to the domestication of silkworms is not surprising, as epigenetics is now a well-established driving force behind phenotype shaping. Nonetheless, the implications are already obvious, even if these findings will need to be further explored and confirmed. For example, can the authors show that bigger salivary glands are capable of producing more silk? What are the other characteristics of domesticated silkworms that were determined by altered epigenetics? Can we go back to wild silkworms by reverting the epigenetic marks of the domesticated worms in the laboratory?

In the past years, genetic engineering of silkworms has been conducted with considerable success, though the applications have not directly impacted on silk manufacturing (the interesting experiments performed in the past included silkworms engineered to produce fluorescent silk or a very strong type of spider silk.

In the future, we may assist to the advent of new silkworm varieties that allow for an increase in silk production with the use of fewer animals. In this way, genetic engineering would have a positive impact on the breeding of silkworms, as one important aspect to take into consideration when harvesting silk should be animal welfare. It is now clear that the new findings should be considered when deciding which genes to engineer: epigenetic modifiers are amongst the obvious candidates.